Known receivers for receiving digital terrestrial transmissions are generally of the single conversion or double conversion superheterodyne type. Although both types of tuner can provide adequate performance, they have features which make it difficult or impossible to increase the degree of integration and minimise the tuner size.
In the case of single conversion tuners, in order to achieve adequate selectivity so as to provide acceptable performance in the presence of potentially interfering signals, a plurality of tracking filters are required. Such filters must be aligned, either during manufacture or by means of some form of automatic calibration during operation. Such filters occupy a substantial area and prevent “upintegration”, because they cannot effectively be integrated in a multi-chip module (MCM) or by integrated circuit techniques. Also, such filters are unsuitable for implementation on a motherboard because of manufacturing problems associated with the requirement for alignment.
Double conversion tuners are more suited to board-level implementation and upintegration because they make use of broadband techniques so that no front-end tracking selectivity filters are required. However, because of the broadband nature of the front-end, it is more difficult to achieve the required dynamic range for providing acceptable performance. Also, an image filter is required at the first intermediate frequency (IF). Such a filter is typically of high Q construction and may be embodied as a surface acoustic wave (SAW) or dielectric filter.
In addition to the image or selectivity filters in both types of tuner, further intermediate frequency filtering has to be provided in order to suppress adequately immediately adjacent channels. Such a filter is typically a SAW device with a passband centred at an intermediate frequency, for example of 36 MHz in the case of UK-based digital transmissions.
GB1594632 discloses a single-conversion superheterodyne receiver for relatively low frequency signals of the type received during prospecting for hydrocarbons. The receiver has a plurality of individually selectable filter paths between an input and a frequency changer. The filter paths are selected according to the frequency decade to be received.